Tabular silver halide grains are crystals possessing two major faces that are substantially parallel. The average diameter of said faces is at least three times the distance separating them (the thickness). This is generally described in the art as an aspect ratio of at least 3:1.
Silver halide photographic emulsions containing a high proportion of tabular grains have advantages of good developability, improved covering power and increased useful adsorption of sensitizing dye per weight of silver due to their high surface area-to-volume ratio. The use of such emulsions in photographic elements is disclosed in U.S. Pat. Nos. 4,425,425, 4,425,426, 4,433,048, 4,435,499, 4,439,520, and other related patents.
The use of automatic processors for the rapid processing (i.e., for a processing of from 45 to 90 sec) of light-sensitive silver halide elements including tabular silver halide grains, in particular light-sensitive silver halide elements for radio-graphic use, is known. Such elements generally include a support (usually provided with a very thin subbing layer) having coated on at least one side thereof a silver halide gelatin emulsion layer coated in turn with a gelatin protective layer. These elements are transported through the machine processing units (developing, fixing, washing and drying) by means of opposed or staggered rollers (as described, for example, in U.S. Pat. No. 3,025,779) which also have the function of squeezing liquid from the film prior to drying. In recent years the increased use of silver halide elements for radiography has led to a strong request for a reduction of processing times. If rapid processing of a film takes place, several problems can occur, such as an inadequate image density (i.e. insufficient sensitivity, contrast and maximum density), insufficient fixing, insufficient washing, and insufficient film drying. Insufficient fixing and washing of a film cause a progressive worsening of the image quality and modification of the silver tone. In order to reduce the time taken by the element to pass through the processing machine from 2 to 0.5 minutes, as particularly required in rapid processing of radiographic elements, the processing is performed at relatively higher temperatures, usually higher than 30.degree. C., preferably between 35.degree.-45.degree. C., such as 38.degree. C., and the gelatin content of the silver halide emulsions is considerably reduced as compared to that of emulsions for manual processing.
Under such conditions, even with the changes in the emulsions, the physical and photographic properties of the elements processed in an automatic processor tend to be worse. With high temperatures and in presence of such low gelatin content, for instance, the intrinsic sensitivity to pressure of the silver halide grains gets higher and the elements processed in the automatic processor show marks caused by the pressure of the transporting rollers. Such pressure marks look like higher density regions and reduce the image faithfulness.
In order to prevent pressure marking, various methods have been described in the art. To this purpose, U.S. Pat. No. 2,960,404 describes the use in the photographic elements of glycerine, ethylene glycol and the like, Japanese Pat. No. 5316/1972 describes the use of 1,4-cyclohexane dimethanol and the like, and Japanese Pat. No. 4939/1978 describes the use of trimethylol propane. Another possible method of preventing pressure marking is by increasing the degree of hardening of the gelatin layers, in particular of the external protective layers. As another method, photographic elements are known wherein an intermediate gelatin layer is interposed between the support and the emulsion layer. For example, U.S. Pat. No. 3,637,389 describes a rapid processing photographic element wherein gradation, density and sensitivity are improved by applying such an intermediate gelatin layer between the support and the emulsion layer.
However, known methods of preventing pressure marking when used in photographic elements including tabular silver halide grains have proved less effective. In particular, when the hardening degree is increased to achieve a very low swelling index and to improve its resistance to pressure desensitization, photographic characteristics are reduced. Accordingly, the problem still remains of preventing pressure marking in photographic elements including light-sensitive tabular silver halide emulsions.
U.S. Pat. No. 4,414,304 describes forehardened photographic elements, particularly radiographic elements, including at least one hydrophilic colloid emulsion layer containing tabular silver halide grains having an aspect ratio of not lower than 5:1 and a projective area of not lower than 50%. The elements require no additional hardening on development and give images of high covering power. Among gelatin hardeners, bis(vinylsulfonylmethyl) ether, mucochloric acid and formaldehyde are described.
Japanese Pat. Appl. No. J5 9105-636 describes photographic elements comprising at least one silver halide emulsion layer containing tabular silver halide grains, the binder of at least one of the hydrophilic colloidal layers being gelatin which has jelly strength of at least 250 g. Wet coat strength of said elements is improved without reducing covering power.
Japanese Pat. Appl. No. J6 2249-140 describes photographic elements comprising at least one silver halide emulsion layer containing tabular silver halide grains and halogen substituted s-triazine type hardeners. The elements are suitable for rapid processing and have improved pressure resistance.
U.S. Pat. No. 4,847,189 describes a photographic element comprising at least one silver halide emulsion layer containing tabular silver halide grains with an aspect ratio not lower than 5:1 and showing a melting time of from 8 to 45 minutes. The melting time and the gelatin amount of the element renders the element suitable for rapid processing of 45 sec. and improves the pressure desensitization resistance.
EP 238,271 discloses a silver halide photographic material comprising at least one hydrophilic colloidal layer on a support, showing a melting time of from 8 to 45 minutes, and a water content of from 10 to 20 g/m.sup.2 upon completion of the washing step. The material is preferably processed in a developing solution comprising indazole and benzotriazole derivatives. The preferred processing time is 45 sec.
U.S. Pat. No. 4,647,528 discloses a method of increasing both covering power and scratch resistance by using a particular polymeric hardener in a photographic material comprising a support coated with at least one silver halide emulsion layer containing tabular silver halide grains with an aspect ratio higher than 5:1.
However, when performing a super-rapid processing of less than 45 sec the above mentioned disadvantages are not necessarily overcome by these techniques, and thus there is still the need for a silver halide photographic material which shows good photographic and physical characteristics when processed in a super-rapid processing.